Ammonium octamolybdate, (NH.sub.4).sub.4 Mo.sub.8 O.sub.26, exists in two isomeric solid forms, designated as alpha or beta forms. Ammonium octamolybdate-alpha, abbreviated AOM-.alpha., is used in plastic formulations as a smoke suppressant and flame retardant. It is made commercially by the partial thermal decomposition of ammonium dimolybdate, (NH.sub.4).sub.2 Mo.sub.2 O.sub.7, abbreviated ADM, according to the chemical equation: ##STR1## However, the commercial process for making AOM-.alpha. by partial thermal decomposition of ADM has two serious disadvantages. The first disadvantage is that the AOM-.alpha. product has too large a particle size; therefore, the particle size must be reduced before it can be used in plastic formulations. The necessity for particle size reduction increases the cost of manufacturing the AOM-.alpha. by a significant amount. In addition, the particle size reduction frequently is not performed adequately, with the result that the AOM-.alpha. does not function properly in the plastic. This causes user dissatisfaction and loss of sales.
The second disadvantage of the present commercial process is the difficulty in controlling thermal decomposition. If the ADM starting material is not heated long enough or hot enough, underreaction will occur and ammonium trimolybdate (NH.sub.4).sub.2 Mo.sub.3 O.sub.10, abbreviated ATM, will be produced. If the ADM starting material is heated too long or too hot, overreaction will occur and molybdic oxide, MoO.sub.3, will be produced. Therefore, great difficulty is experienced in producing AOM-.alpha. which is not contaminated with significant amounts of ATM or MoO.sub.3. In order to avoid underreaction or overreaction, a very uniform heating rate is required which is difficult to achieve when solid particulate material is being heated since conductivity is poor. In other words, the temperature must be the same everywhere and throughout the bulk of the solids at any given time. In addition, a very precise temperature and time control is required. In order to achieve the necessary heating control, thermal decomposition must be conducted on a small scale, resulting in a very labor-intensive process. In actual fact, the commercial process is conducted by heating the ADM in flat pans placed in a box oven with loading and emptying performed by hand.
The present invention overcomes the disadvantages of the commercial process described in the preceding paragraphs by producing a much finer AOM-.alpha. by a process which is much easier to control. Additionally, AOM-.alpha. with the desired purity and fine particle size can be achieved with less effort and with greater reliability.